Corticomotoneuronal (CM) and rubromotoneuronal (RM) cells terminate preferentially on motoneurons of distal muscles and represent the major descending systems in primates for skilled use of the limbs. Understanding the mechanism of action and functional role in movement of CM and RM neurons requires a knowledge of their terminal distribution patterns with motoneurons, their discharge during active movements, and their responses to cutaneous and kinesthetic stimuli. I propose to use spike-triggered averaging (SpTA) of rectified EMG activity to investigate the effect of single wrist-movement related CM cells on the activity of 12 covarying flexor and extensor muscles. Specifically, I will determine whether CM cells showing strict reciprocal activity during alternating wrist movements also produce reciprocal effects on motoneurons such that agonists are facilitated and antagonists suppressed. SpTA will also test whether CM cells related to coactivation of wrist flexor and extensor muscles also facilitate both these muscle groups or only one or the other. The terminal organization of wrist-movement related RM cells will be determined by computing SpTAs of 12 covarying flexor and extensor muscles. Just as SpTA reveals the terminal distribution pattern of a group of neighboring cells, so stimulus-triggered averaging at te same site will test the extent to which neighboring RM cells have similar terminal distribution patterns. To investigate the encoding of movement parameters, particularly static torque, in the discharge of RM cells their firing rates will be recorded for different concentric and isometric loads. Comparison with similarly determined relationships for CM cells will reveal the relative contribution of each system to the generation of static torque. Last, I will investigate the role of RM neurons in the compensatory recovery of motor function following destruction of the CM system by comparing (1) the strength and distribution of post-spike facilitation, and (2) the relation between RM cell firing rate and static torque before and after pyramidal tract section.